Conventionally, a communication apparatus is mounted on a vehicle. The apparatus includes multiple antennas, each of which receives an electric wave. The apparatus selects one of antennas, which receives the wave with high quality, so that the apparatus provides a diversity antenna system.
When a communication apparatus only includes one antenna, the antenna has directionality so that communication quality is improved. This technique is disclosed in JP-A-2006-314071 corresponding to US 2008/0291097.
However, in the diversity antenna system, it is necessary to include multiple antennas, so that dimensions of the apparatus become large. Further, in the technique disclosed in JP-A-2006-314071, although the dimensions of the apparatus are small, the communication may be interrupted when the apparatus is disposed at a dip point, at which receiving electric power is much reduced. Here, at the dip point, the receiving electric power is reduced by influence of multiple paths caused by reflection of the electric wave on a road surface or a wall of a building in a city since the antenna has the directionality.
Thus, it is required to provide a communication apparatus with small dimensions and without receiving influence of a dip point.